(1) Field of the Invention
The present invention relates to a structure of an oxygen sensor for detecting the concentration of oxygen in the exhaust gases from various combustion apparatus.
The present invention is also concerned with a structure of an oxygen sensor equipped with a heater for heating an oxygen sensing element.
(2) Description of the Prior Art
Oxygen sensors using an oxygen sensing element comprising a solid electrolyte such as zirconia have heretofore been widely used for detecting the concentration of oxygen in the exhaust gases from various combustion apparatus including internal combustion engines.
Oxygen sensors of this type, as shown in FIG. 1, are composed of a cylindrical oxygen sensing element 1 formed of zirconia or the like and having one closed end, inner and outer electrode layers 2 and 3 formed of platinum or the like deposited on the inner and outer surfaces of the oxygen sensing element 1, an output withdrawing lead wire 4 connected electrically to the inner electrode layer 2 for withdrawing output from the same layer, and a housing 5 for holding the oxygen sensing element 1, the housing 5 being connected electrically to the outer electrode layer 3, in which the inner and outer electrode layers 2 and 3 are contacted with ambient air and exhaust gases, respectively, whereby the concentration of oxygen in the exhaust gases is sensed.
Thus, it is necessary that the inner electrode layer 2 be contacted with ambient air and at the same time connected to the output withdrawing lead wire 4. But, various difficulties have therefore been encountered in the connection between the output withdrawing lead wire 4 and the inner electrode layer 4. For example, an internal thread portion 6 is formed in the inner surface of an open end portion of the oxygen sensing element 1, and a tubular metallic terminal 8 having an external thread portion 7 is engaged with the internal thread portion 6 and at the same time connected to the output withdrawing lead wire 4, or a coil-like terminal is engaged with the internal thread portion and at the same time connected to the output withdrawing lead wire, to thereby electrically connect the inner electrode layer 2 and the output withdrawing lead wire 4 with each other. In these cases, however, a large clamping stress may be exerted on the internal thread portion 6 thereby causing damage of the oxygen sensing element 1. It has also been tried to prevent damage of the oxygen sensing element by using a coiled spring terminal in place of the cylindrical metallic terminal 8 and fitting it threadedly in the internal thread portion 6. But, in this case, since the outside diameter of the coiled spring terminal is formed larger than the inside diameter of the internal thread portion 6 so as to press the inner periphery of the internal thread portion 6, it is not easy to bring the coiled spring terminal into engagement with the internal thread portion 6, the operation being troublesome. Further, in view of the characteristic of the oxygen sensing element that this sensing element is improved in its oxygen sensing performance while it is held at a predetermined temperature, there has been proposed an oxygen sensor incorporating a heater for heating the oxygen sensing element. In this case, however, it is necessary to dispose such heater in the interior of the oxygen sensing element, that is, inside the inner electrode layer, so particularly in the case of using a rod-like ceramic heater as such heater, a problem is encountered in the fixing method for the heater body. And in the case of using a coil-like terminal, there will not occur breakage of the oxygen sensing element because a clamping stress is not applied thereto, but it is not easy and is troublesome to fit such terminal threadedly in the internal thread portion.
Moreover, the heater is fixed using an adhesive or the like to heat the oxygen sensing element. But, in this case, the heater cannot be fixed firmly due to a change of the adhesive or the like with the lapse of time caused by heating or vibration, thus resulting in damage of the inner electrode layer or the heater itself.